1. Field of the Invention
The present invention relates to an electrically conducting polymer, a method for preparing the same and an electrolytic capacitor comprising the same which has improved frequency characteristics and long-time stability.
2. Description of the Related Art
An electrically conducting polymer is a polymer having a widely spread conjugated .pi. electron system such as polypyrrole, polythiophene, polyaniline, polyacene and the like and has good electrical conductivity when the polymer contains an electron-donor or an electron-acceptor (e.g. an anion of Lewis acids or protonic acids) as a dopant. However, the conventional dopants have a drawback that the molecules of the dopant diffuse in the polymer matrix due to application of an electric field so that a conductivity of the electrically conducting polymer is decreased.
In view of this drawback of the dopant, it is proposed to use, as a dopant, a compound having a larger molecular weight such as phthalocyaninetetrasulfonic aid and polystyrenesulfonic acid (cf. K. Okabayashi, J. Chem. Sci., Chem. Commun., 684 (1983) and N. Bates et al, J. Chem. Sci., Chem. Commun., 871 (1985)). However, these compounds are bulky, conductivity of the polymer containing them is not so high as the polymer containing the low molecular weight dopant.
As described above, when the low molecular weight dopant is used, the electrically conducting polymer has higher conductivity although the molecules of dopant move in the polymer matrix under the application of direct current electric field, whereby the conductivity in the polymer may have very large irregularity in its distribution. On the other hand, when the bulky dopant is used, the conductivity of the conducting polymer cannot be made high. When such electrically conducting polymer is used in an electrolytic capacitor, impedance in pores at a surface of a dielectric layer cannot be kept low for a long time, and one of the advantages of the electrolytic capacitor comprising the electrically conducting polymer, namely good frequency characteristics, is deteriorated.